1. Field of the Invention
The present invention relates to a method and apparatus for Power Headroom Reporting, and more particularly, to a method and apparatus for performing Power Headroom Reporting to support carrier aggregation in a user equipment (UE) of a wireless communication system.
2. Description of the Prior Art
Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in base stations (Node Bs) alone rather than in Node Bs and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
In LTE system, a MAC Protocol Data Unit (PDU) consists of a MAC header, zero or more MAC Service Data Units (SDUs), zero or more MAC control elements, and optionally padding. A MAC PDU header consists of one or more MAC PDU sub-headers, each corresponding to either a MAC SDU, a MAC control element or padding. The MAC PDU sub-headers have the same order as the corresponding MAC SDUs, MAC control elements and padding.
According to the current specifications (3GPP TS 36.321, 36.213, 36.133), MAC control elements transmitted by a UE include a buffer status report (BSR) MAC control element and a power headroom report (PHR) MAC control element. The BSR MAC control element is generated by a Buffer Status reporting procedure, and is used to provide the serving base station, or called enhanced Node B (eNB), with information about the amount of data in the uplink (UL) buffers of a UE for scheduling of uplink transmission. The PHR MAC control element is generated by a Power Headroom reporting procedure, and is used to provide the serving eNB with information about the difference between the maximum UE transmission (TX) power and an estimated TX power for Uplink Share Channel (UL-SCH). With the BSR and PHR information sent by the UE, the network can allocate radio resource to the UE and make schedule decision more efficiently.
In general, a PHR is triggered if any of the following events occurs: (1) a timer “prohibitPHR-TIMER” for prohibiting power headroom reporting expires or has expired and a path loss of the UE has changed more than a predetermined value “DL_PathlossChange” since the last power headroom report; and (2) a periodic timer “PeriodicPHR-Timer” expires, in which case the PHR is referred below to as “Periodic PHR”. After the PHR is triggered, if the UE has UL resources allocated for a new transmission for this TTI, the UE obtains the value of the power headroom from the physical layer to generate a PHR MAC control element, and restarts the timer “prohibitPHR-TIMER”. Besides, if the triggered PHR is a “Periodic PHR”, the UE restarts the periodic timer “PeriodicPHR-Timer”. Detailed operations of the PHR procedure can be referred to in related specifications (3GPP TS 36.321, 36.213, 36.133) and are not narrated herein.
On the other hand, the 3rd Generation Partnership Project (3GPP) has started to work out a next generation of the LTE system: the LTE Advanced (LTE-A) system, to meet future requirements of all kinds of communication services. Carrier Aggregation (CA) is introduced in the LTE-A system, which enables the UE to aggregate multiple carries for transmission, such that the transmission bandwidth and spectrum efficiency can be enhanced.
At present, the characteristics of carrier aggregation are quoted as below:
(1) Supporting carrier aggregation for both contiguous and non-contiguous component carriers.
(2) The aggregated number of component carriers in the uplink (UL) and the downlink (DL) can be different. For backward-compatible configuration, the aggregated numbers of component carriers in the UL and the DL should be the same.
(3) It is possible to configure a UE to aggregate a different number of component carriers in the UL and the DL to obtain different bandwidths.
(4) From a UE perspective, there is one transport block and one hybrid-ARQ (HARQ) entity per scheduled component carrier. Each transport block is mapped to a single component carrier only.
However, after the carrier aggregation is introduced, how to report the PHR value for different carriers and how to trigger the power headroom reporting are not specified. When multiple carriers are configured for transmission (due to the support of CA), if only one PHR value is reported by the UE, the eNB cannot know the power status of each uplink carrier of the UE and thus is hard to schedule radio resources to the UE efficiently. Therefore, there is a need to have the PHR procedure under carrier aggregation be specified.